This invention relates to a fuel vapor handling system. More particularly, the invention is directed to an apparatus and method for use in diagnosing the system.
Automobiles conventionally include a system directed to controlling the emission of fuel vapors generated by the fuel carried in the fuel system. These evaporative emission control systems, known as "EVAP" systems, are implemented as a collateral system to the vehicle's fuel system. The diurnal and running loss vapors they collect result primarily from ambient temperature excursions and the cyclic operation and parking of the vehicle that results from the operator's use of the vehicle as transportation.
An EVAP system typically includes a vapor collection system with an adsorption mechanism to collect and store vapors generated by the fuel system. The EVAP system also includes a purge system to transfer the stored fuel vapors from the adsorbent to the vehicle's engine for consumption in the normal combustion process. The purge system generally includes a normally closed purge valve that selectively opens a passage between the EVAP system and the vehicle's engine to effect a controllable rate of purge.
Conventionally, diagnosis of an EVAP system is generally provided through manual inspection of the system in response to noticeable engine performance degradation or noticeable fuel or vapor leakage. Periodic manual vacuum testing for leaks and purge valve functional checking provides additional effectiveness in diagnosing system operation.
Research has been conducted into developing on-board means for automatically diagnosing EVAP systems, capable of automatically detecting leaks in the system and determining whether the vapor collection and purge systems are operating correctly. Development of on-board automatic diagnostic systems has generally resulted in proposed systems related to mechanisms that close the EVAP system off from the atmosphere and then generate a positive or negative internal system pressure. By then measuring changes in the system pressure, the diagnostic mechanisms attempt to discern whether the evaporative control system is functioning correctly.
Generally, sensitive diagnostic systems are proposed with precision pressure detection devices to work on small pressure differentials. To avoid unacceptable erroneous fault reporting, a pressure based diagnostic system must be able to discern that unexpected pressure gradients are a result of system malfunctions and not changing ambient conditions or other normal collateral effects. This tends to complicate and drive up the cost of a diagnostic mechanism. Accordingly, automatic EVAP diagnostic systems have proven difficult to implement.
Adsorption canister collection and storage system use in on-board refueling vapor recovery (ORVR) systems is known. ORVR systems are vehicle based systems directed at capturing fuel vapors generated by the transfer of fuel from a pump to a vehicle. ORVR systems have been proposed that are configured in a manner similar to an EVAP system including a storage canister and a purge system. Therefore, an automatic system capable of diagnosing an ORVR system is also needed.